U.S. Pat. No. 5,193,512 discloses a tank-venting system which has a tank with a tank-pressure sensor, an adsorption filter connected to the tank via a tank-connecting line, and a tank-venting valve which is connected to the adsorption filter via a valve line, in which system the adsorption filter has a venting line which can be closed by means of a shut-off valve. The tank-venting system configured in this way is checked for operability by the following method:
checking to determine whether an operating state is present, for example full load, in which no significant underpressure can build up in the tank after the closure of the shut-off valve and the opening of the tank-venting valve;
if such a state is present, the method is terminated; otherwise, the following steps follow:
closing the shut-off valve;
opening the tank-venting valve;
measuring of the underpressure building up in the tank; and,
evaluating of the tank-venting system as non-operative if a pregiven underpressure is not reached.
U.S. Pat. No. 5,205,263 discloses a method which operates on a tank-venting system without a shut-off valve and has the following method steps:
opening the tank-venting valve;
determining the build-up gradient of the underpressure building up in the tank; and,
comparing the build-up gradient and/or the decay gradient with a respective threshold value and evaluation of the system as operative if the at least one gradient and the corresponding threshold value fulfill a pregiven relationship.
U.S. patent application Ser. No. 08/070,334, filed May 26, 1993, discloses a similar method which is however carried out on a tank-venting system with a shut-off valve. Measurements for determining the build-up and the decay gradient are only taken into account once it has been ensured that the measurements are not influenced by vaporizing fuel. For this purpose, a leanness correction check is used with the aid of a lambda controller and/or a check as to whether the vehicle and hence also the contents of the tank are presumably in motion.
It has been shown that the known and proposed methods require further refinement in order to be able to detect small leaks in the order of 2 mm.